FIG. 14 illustrates a control block diagram of a conventional magnetic recording apparatus.
As illustrated in FIG. 14, for example, a magnetic recording apparatus 100 includes a magnetic recording medium 130 of a magnetic disk, a magnetic head 110 that performs recording and reproduction, a head positioning mechanism (VCM) 111 that positions the magnetic head 110, a read amplifier 112 that makes a signal from a reproduction device of the magnetic head 110 a reproduction signal, a write amplifier 113 that drives a recording element of the magnetic head 110 according to a write signal, a disk controller circuit 114, a read channel circuit 115, a positioning control circuit 116, a driving circuit (power amplifier) 117, and an oscillator 120. The disk controller circuit 114 receives a record or reproduction command from a host computer 140, calculates and controls a signal to be recorded in the magnetic recording medium 130, and returns a reproduced data signal from the magnetic recording medium 130 to the host computer 140 as a data bit string. The read channel circuit 115 demodulates the reproduction signal from the magnetic recording medium 130 to generate a data signal or a servo signal, and modulates the data signal received from the disk controller circuit 114 in a manner appropriate for recording in the magnetic recording medium 130, to generate a record signal which synchronizes with a write clock pulse. The positioning control circuit 116 outputs a control signal for positioning the magnetic head 110 based on the servo signal which is demodulated in the read channel circuit 115. The driving circuit 117 converts a drive signal outputted from the positioning control circuit 116 into a current, to drive the head positioning mechanism 111. The oscillator 120 supplies the read channel circuit 115 with the write clock pulse.
In the above conventional practice, the write clock pulse that gives a timing to operate the write amplifier 113 is typically generated by using the oscillator 120 with a fixed frequency, and supplied to the read channel circuit 115.
The trend toward smaller size of a magnetic disk apparatus is apparent in recent years. With this trend, there is a demand for the development of a high-density recordable disk medium. However, in the improvement of the recording density of the magnetic disk apparatus, it becomes a problem to prevent interference from adjacent recording bits.
Under recognition of such problem, a technique called discrete track recording is proposed, which is a technique of physically sectioning each record track of the magnetic disk medium, against interference in a diametrical direction of the magnetic disk medium, to reduce the interference from adjacent tracks.
Further, under the recognition of the above problem, a technique called “patterned media” is also proposed, which is a technique of also physically sectioning the medium, in other words, a technique of patterning each recording bit, against interference in a rotating direction of the magnetic disk medium, to reduce the interference from adjacent bits.
Even when the problem to prevent the interference from adjacent recording bits can be solved, it is necessary for the practical use thereof to accurately position a magnetic head on the medium which is made finer. It is therefore necessary to overcome both the problem to prevent the interference from adjacent recording bits and the problem to accurately position the magnetic head on the medium which is made finer.
Particularly, with regard to positioning in the rotating direction, in other words, means for accurately performing recording or reproduction at the center of each of the sectioned media, it is necessary to detect or predict a timing of passage of the recording and reproduction head over a magnetic body region, and to generate a clock signal which synchronizes with an arrangement pattern of the magnetic body region.
As a technique for generating this clock signal, for example, Patent Document 1 proposes a magnetic recording apparatus that detects a leakage magnetic field generated in recording in a non-magnetic body region, and correct a phase shift of the clock signal.
Further, Patent Document 2 proposes a magnetic recording apparatus that performs recording and reproduction while shifting a phase of a clock signal, and select a phase with which the most favorable error rate is obtained.
Patent Document 1: Japanese Laid-Open Patent Publication No. 2003-281701
Patent Document 2: Japanese Laid-Open Patent Publication No. 2006-164349
For generation of the clock signal which synchronizes with the arrangement pattern of each of magnetic body regions, for example, according to the Patent Document 1, a leakage magnetic field generated in recording in each of non-magnetic body regions is detected to correct a phase shift of the clock signal. However, the method described in Patent Document 1 has a problem that it is impossible to compensate a phase shift when the leakage magnetic field is not accurately detected.
Further, the method of Patent Document 2 has a problem as follows. It is generally required for a recording operation in the magnetic recording apparatus to perform promptly after reception of a recording command from a host computer. However, in the method of Patent Document 2, an optimum phase shift amount is obtained by a trial by measurement of an error rate, so that it is impossible to perform the recording operation immediately after the reception of the recording command. And, it is needed for avoiding this to hold a previously measured phase shift amount in a memory, and reuses it when the apparatus is operated.
However, the magnetic recording mediums mounted in the apparatus are often clamped by frictional force to a turn table of a spindle motor that rotates the medium. In this case, the magnetic recording medium may rotate against the turn table due to impact of some kind to cause a change in optimal phase shift amount. As a result, the phase shift amount cannot be reused which is previously measured and held in the memory. Further, the optimal phase shift amount itself may be changed by mechanical shrinkage and expansion of the medium due to a change of environmental temperature.
In the above circumstances, the phase shift amount is to be repeatedly measured, so that significant deterioration in performance of the apparatus is caused in the method described in Patent Document 2.
Further, due to an influence of a change in decentering of the magnetic recording medium, a rotational jitter of the turn table, or the like, a frequency of the clock signal may be shifted from an estimated value.
Each of the methods described in Patent Document 1 and Patent Document 2 is a method for detecting a phase shift, but a method for detecting a frequency shift is not referred to therein.